1. Field of the Invention
The disclosure relates to an optical system, such as, for example, an illumination system or a projection lens of a microlithographic exposure system.
2. Description of the Related Art
Microlithography is used in the fabrication of microstructured components like integrated circuits, LCD's and other microstructured devices. The microlithographic process is performed in a so-called microlithographic exposure system comprising an illumination system and a projection lens. The image of a mask (or reticle) being illuminated by the illumination system is projected, through the projection lens, onto a resist-covered substrate, typically a silicon wafer bearing one or more light-sensitive layers and being provided in the image plane of the projection lens, in order to transfer the circuit pattern onto the light-sensitive layers on the wafer.
In order to obtain a good interference contrast of the image created in the image plane of the projection lens, it can be desirable to have a two-ray-interference of light rays each being polarized perpendicular to the plane of incidence.
In the context of the present application, the polarization component that oscillates perpendicular to the plane of incidence of a light ray is referred to as s-polarized component, while the polarization component that oscillates parallel to the plane of incidence of a light ray is referred to as p-polarized component. Furthermore, a polarization distribution where the planes of oscillation of the electrical field vectors of individual linearly polarized light rays in a pupil plane of the system have an orientation that is perpendicular to the radius originating from the optical axis will hereinafter be referred to as tangential polarization. A polarization distribution where the planes of oscillation of the electrical field vectors of individual linearly polarized light rays in a pupil plane of the optical system have a radial orientation relative to the optical axis will hereinafter be referred to as radial polarization.
Various approaches to create an at least approximate tangential polarization distribution in an illumination system or a projection lens are known in the art. It is, however, a problem to bring (or “transport”) such a once created tangential polarization state to the image plane of the projection lens, which is due to various depolarizing or polarization-modifying effects arising from e.g. the microstructures in the reticle, from phase shifts occurring at reflecting elements or from natural, intrinsic or stress-induced birefringence in the optical elements.